Pressures exerted by the expansion of freezing water within duct or conduit installations and their associated vaults or enclosures can be extreme. These pressures have been calculated to reach upward of 60,000 psi, which is equivalent to the pressures commonly encountered in large caliber rifle chambers when firing a cartridge.
Utilities in northern temperate, sub-arctic and arctic regions such as the northern contiguous United States, Canada and Alaska (and other similar regions worldwide) have tried for many years to devise techniques to prevent damage to power and communication conductors and equipment that are installed within duct/vault systems.
Generally, conductors are installed in conduits or duct systems for mechanical protection from the environment. Ducts and conduits allow for possible replacement after a conductor failure when the ground is frozen or without disturbing the surface area above the duct or conduit. Conductors installed in underground ducts are generally classified as being installed in wet locations. However, electrical rigid metal conduits do not have tapered threads and when installed, are not watertight.
While the air pressure within installed conduits and ducts is basically 0 psi gauge, ground water pressure is always higher. Water will force in through couplings, expansion joints or other duct connections. Water also enters ducts, vaults or enclosures by infiltration or flooding from the surface or will flood in or infiltrate through the open conduit ends. Once in the conduit or duct system, water fills the voids between the conductors within the conduit.
During winter months surrounding ground freezes down to or beyond a depth of 6 to 7 feet depending upon geographic location. Conduits are typically placed from 24 to 42 inches below grade, which is well within the freeze depth. As the ground freezes around the conduit, it forms a layer of frozen soil around the conduit that can approach the strength of concrete. As the ground continues to freeze, the water at the ends and inside the conduit also starts to freeze, capturing liquid water in the conduit.
As water continues to freeze in these confined spaces, the pressure increases due to the expansion of water as it changes to ice. If the conduit/duct is above ground, the conduit will rupture from the high pressure. When the conduit is in frozen ground the strength of the conduit is greatly increased by the surrounding frozen earth which allows the pressure inside the conduit to reach extremely high pressures. As these high pressures increase, the pressure is applied to the conductors, which causes deformation and failure of the conductor insulation. Driven by increasing pressure, expanding ice (which is bonded to the conductor insulation) attempts to flow along the duct or conduit seeking the necessary volume dictated by its change of state from water to ice. At typical pressure, that necessary additional volume can only be found at the conduit or duct ends of the installed system, which results in insulation and/or conductor failure.
Some techniques have been attempted to protect conductors in conduits or ducts located within frozen ground or free air from damage caused by the expanding frozen water. These techniques range from keeping the water out, using heat and other chemicals, and displacing the water with another material.
Attempting to keep the water out is commonly called the submarine approach. Keeping water out of a conduit system can be extremely difficult unless all water entry points are sealed and continuous maintenance methods are strictly assured and enforced. However, couplings on rigid metal conduits are not sealed and allow water entry from the elevated water pressure that exists around a buried conduit. Additionally, above ground ducts/conduit systems also tend to retain all infiltrated water. The most common way to avoid standing water in conduits is grading, where the conduit is sloped to a drain point. However, in areas of high water table, the drain point allows water to flow back in the conduit/duct from the intended drain point. The layout of the conduit/duct can also interfere with draining when there are elbows or fittings that are intended to provide a continuous enclosed path from buried depth to the surface. Additionally, storming conditions or flooding can allow water to enter conduits/ducts from their end points.
Keeping the water out through the use of heat or chemicals is also not practical and does not work. Heat and chemicals are expensive and often impractical or wasteful. Chemicals can be added to the conduit to suppress the freezing point of the water, similar to anti-freeze. However, chemicals must be approved for use with the conductor insulation and monitored against dilution over time must be assured. Further, with heated ducts/conduits temperatures must be controlled and monitored to prevent insulation damage and allow the full capacity of the conductor to be achieved.
Displacing the freezing water with another material, such as expanded or blown in beaded foam, has been tried. Expanding foam tends to expand around the conductors and will prevent the change out of the conductor following a failure. Beaded foam will displace the water but will not withstand flowing water which can occur in a conduit/duct.
Thus, the need exists for solutions to the above problems with the prior art.